1. Field of the Invention
The present invention relates to vacuum packages, and particularly to multiple internal seal ring (MISR) micro-electro-mechanical (MEMS) vacuum packages.
2. Description of the Related Art
It is well known in the art to fabricate micro-electro-mechanical system (MEMS) devices. MEMS devices typically integrate both electronic and mechanical elements on a semiconductor wafer using semiconductor fabrication techniques. Generally, the electronic and mechanical elements are fabricated using a variety of integrated circuit (IC) processing and micromachining techniques, respectively.
In the fabrication of MEMS devices, it is not unusual to package or “cap” such devices to prevent damage or contamination to the MEMS devices. Often, the cap comprises a capping plate fabricated from a capping wafer that is bonded to a base plate before the base plate wafer is diced or singulated into individual devices. The capping plate thus hermetically seals the MEMS devices within the base plate.
Even though sealed by the capping plate, the MEMS devices require electrical connection to external circuits. However, the traces used for connection may create gaps where the capping plate and base plate are bonded. Generally, the surfaces where the capping plate wafer and the base plate wafer are bonded must be ultra-flat, i.e., polished. This flatness requirement is difficult to meet if additional processing steps are required for bringing electrical feed-through metal traces out through the bonded surfaces. Specifically, bringing electrical wires out through the bonded surfaces requires metal patterning and oxide growth on the wafers' surfaces. Achieving a bond or seal on top of these uneven surfaces is much more difficult.
Accordingly, what is needed, therefore, is an improved method for sealing and packaging MEMS devices that allows for electrical connection to external circuits.